I'm a British citizen who is normally resident in the US and my health insurance through Premera (an individual plan, not a group plan or through my employer) is a very cushy plan with solid international coverage through Blue Cross / Blue Shield's "Blue Card" programme (provided by AXA, I believe).

As I'm no-longer considered a British resident (and neither pay any UK National Insurance contributions) I believe I'm not eligible for free-at-the-point-of-delivery healthcare, including emergency treatment; but the UK NHS accepts Blue Card insurance so I wouldn't be facing a crippling hospital bill if I were to be hit by a London double-decker bus tomorrow.

I'm currently visiting my parents in the UK and I'll be returning to the US in mid-January.

As it so happens, starting on 1st of January 2017, Premera has decided to eliminate my entire healthcare plan, and also raise insurance premiums by 25% on top of what I'm already paying for the next-closest plan. Among the regressive changes with the new plan is the removal of international health insurance. This is a problem: I'll be uninsured in the UK after midnight US Pacific time on New Years Day.

(This is likely due to the ACA's knock-on effects - but please make no political "thanks, Obama" comments).

So far from what I can tell travel insurance providers require you to buy coverage before you leave the country, sometimes as much as a week in advance.

What options do I have for buying insurance for a couple of weeks while I'm already out of the country?

Have you asked if you can extend your coverage by a month, on a plan that does include international?
– DJClayworthDec 23 '16 at 18:30

@DJClayworth I did ask them over the phone when I renewed for the new plan, they said they couldn't, and now Premera doesn't offer any Individual plans with international coverage.
– DaiDec 23 '16 at 18:38

Do you actually want the insurance or are you merely afraid of violating some rules concerning insurance for tourists?
– JonathanReez♦Dec 23 '16 at 21:28

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@JonathanReez I would like healthcare coverage, yes. My budget is a couple hundred US dollars. I just need emergency care for now.
– DaiDec 23 '16 at 22:04

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A quick search (in German, apologies) leads me to this link which lists two insurers where it seems possible to get coverage after starting your journey (excluding health issues already present). Not sure whether they would cover you as a US resident / UK citizen, but it might be a start.
– mtsDec 24 '16 at 9:54

They all look to be brokers who should have access to a range of insurers. Brokers take a percentage fee, and you should get a quote from more than one broker and ask them to match/beat each other's quotes.

Having spent about 10 years working at one of those places that handle emergency calls and case handling for travel insurance customers, I can categorically state that this answer gives VERY BAD ADVICE. I have witnessed too many instances of families having to mortgage their houses and heavily indebting themselves in order to be able to pay for medical treatment, all because someone went on a trip without adequate insurance. This is not an either/or situation; immediate insurance should be secured as well as appropriate longterm insurance.
– Janus Bahs JacquetDec 25 '16 at 12:43

@JanusBahsJacquet -- why are you vitally concerned about people mortgaging their homes but not about people who just die? That is a very likely outcome of a bus accident but you have not a peep to say. 800 US tourists die abroad every year, but you don't care. You're afraid of... a home mortgage? Yes, if you are in a situation where you can easily get insurance, that's probably wise, better a $50 policy that is only actuarily worth $30, than a long-shot $100,000 expense if you don't have that money -- but that isn't the situation.
– MalvolioDec 25 '16 at 16:00

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Who said anything about not caring about people who die? A good insurance policy will also help with practical and financial matters in the case of death, but as far as the actual injury or death sustained, whether while travelling or not, insurance is of course utterly irrelevant. I completely fail to see what kind of twisted logic can possibly equate having insurance with not caring about people dying. There is always a risk of being hit by a bus and getting killed, as soon as you step out into the world. That risk never, in any way, invalidates the many reasons to buy insurance.
– Janus Bahs JacquetDec 25 '16 at 16:35

I might ask the reverse of you: why are you so vitally concerned about the 800 US tourists who die abroad every year (and yet seem to be actively discouraging them to have an insurance that will help their bereaved ones repatriate their remains and handle stressful international red tape), but seemingly don’t care about people whose lives are essentially ruined by injuries that would have been covered if they had had insurance?
– Janus Bahs JacquetDec 25 '16 at 16:40

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What's improbable about a family living in their own house in a rural area, getting by mainly on government support after the parents lost their jobs, and having, say, a son who goes backpacking during a gap year? That's not at all uncommon in my experience (granted, that’s European, not American). Running up hundreds of thousands or even millions in unexpected medical expenses is rare, but completely unrelated to status or wealth. The maths behind lotteries and insurances is the same; but not winning the lottery doesn't leave you off more than marginally worse than you were to begin with.
– Janus Bahs JacquetDec 26 '16 at 0:05